The glass manufacturing process generally includes a series of successive, interrelated steps each of which takes place at a respective station within the glass manufacturing facility. More particularly, the usual glass manufacturing process includes the steps of drawing a sheet of glass from a bath of molten glass, conveying the molten glass sheet through an annealing furnace with a decreasing temperature gradient over its length that allows the sheet to cool slowly to prevent the buildup of compressive forces within the glass; cutting the annealed glass sheet to the shape and size desired, and tempering the cut sheet by a heating and sudden cooling process to give the glass sheet high compressive forces at its surfaces to minimize susceptibility to breakage and shattering.
The glass material must be conveyed through each of the successive stations at a controlled rate to optimize the quality of the resultant glass product. One known means for conveying the glass sheets between stations and within each station is taught in U.S. Pat. No. 4,133,667 issued to Nitschke. Nitschke '667 discloses a conveyor drive mechanism which includes first and second conveyor drives disposed on opposite lateral sides of a conveyor which includes a plurality of elongate rollers that extend between the conveyor drives and have their opposed ends supported on and in frictional engagement with the conveyor drives. Each of the conveyor drives includes first and second pulleys or sprockets, and a continuous drive loop trained thereover. The glass sheets are supported by the conveyor rollers. A first torque source applies drive torque to the first pulleys, and a second counter-torque source applies a counter-torque to the second pulleys. The cooperative effect of the first and second sources purportedly provides at least a minimum, predetermined level of tension in the active area of the continuous drive loops at all times, to minimize slack and vibration in the drive chain, to thereby minimize slip-stick friction between the glass sheets and the conveyor rollers. Motion is imparted to the conveyor rollers in either a forward or reverse direction by applying a net torque to the rollers by applying a greater drive torque with either the first or second torque sources to the first or second pulleys, respectively, whereby the glass sheets are thusly moved by the conveyor rollers in the selected direction. Nitschke '667 employs a complex control circuit for controlling the energization of the first torque source and the second counter-torque source through a range of net torques.
Nitschke teaches an improvement over his above-discussed conveyor drive mechanism in his U.S. Pat. No. 4,233,053. Nitschke '503 eliminates the control circuit and counterpoised torque sources of Nitschke '667. Nitschke '053 teaches a continuous drive loop trained over the pair of pulleys, so that the rotational motion of the pulleys is imparted to the conveyor rollers, just as in Nitschke '667, except that in Nitschke '053 one pulley is driven by a motor to transport the glass sheets over the conveyor rollers, and the other pulley drives an electrical generator connected to a dissipative load. The generator and load provide an adaptive counter-torque to the other pulley in direct relation to its rotational speed. However, the Nitschke '053 conveyor drive system only provides tension in the active area of the drive loop in a single direction. This is necessarily so, because when the direction of the motor is reversed from that shown in the Nitschke '053 patent, the active area of the drive loop is put into compression, rather than tension, which increases rather than decreases the slack in the active area of the drive loop, which can cause backlash and vibration of the drive loop, which can skew or damage the moving glass sheets. Therefore, Nitschke '053 is limited in application to unidirectional as opposed to bidirectional or oscillating conveyor drives.
Therefore, it would be advantageous to have a conveyor drive mechanism which continuously maintains a predetermined level of tension in the drive loop active area during forward and reverse modes of operation, and which is simpler and more economical in design, construction, operation, and maintenance than the presently available conveyor drive mechanisms.